Torsion damping mechanisms have long been used to reduce the adverse effects of. torsionals or fluctuating torques in vehicle drivelines. Such torsionals or fluctuating torques emanate primarily from engine power pulses and torque spikes, and from abrupt changes in driveline torque due primarily to rapid engine acceleration/deceleration and transmission ratio changes. Effective damping of such torsionals has become increasingly more difficult due to current development trends necessitated by a need to improve vehicle efficiency. The need for improved vehicle efficiency has resulted in reductions in vehicle size and weight, reductions in inertia of driveline components such as flywheel masses, reductions in the number of engine cylinders or chambers, reductions in engine speed, increases in the number of transmission gear ratios, reductions in transmission oil viscosity, and increased use of torque converter bypass clutches.
These developments have dramatically increased existing problems with transmission gear rattle noise, vehicle body noise, and vehicle jerk. Body noise or body boom, as it is sometimes referred to, often occurs when an engine is lugged; under such a condition, engine torsionals cause body components, such as sheet metal panels, to resonate. Vehicle jerk, known as tip-in/tipout, occurs in response to abrupt engine acceleration/deceleration and ratio changes.
Gear rattle, which may be idle rattle and/or in-gear rattle, occurs when unloaded tooth flanks of meshed gears bounce against each other. Both gear rattle problems can be particularly acute when engine torsionals or power pulse frequencies and their harmonics are the same as resonant frequencies of the transmission gear ratios. Idle rattle occurs when a manual transmission is in neutral with the transmission input shaft clutched to an engine running at or near its idle setting, i.e., when engine speed and torque are low or minimum. In-gear rattle occurs when the transmission is in gear (usually low speed gears) and clutched to an engine running above its idle setting, i.e., when engine speed and torque are relatively low but sufficient to drive a load or vehicle.
The prior art is replete with mechanisms of negating or mitigating both forms of gear rattle, body boom, and vehicle jerk. Such mechanisms are commonly incorporated in master clutch plates and, of late, in so called two mass flywheels. It is also known to incorporate a mechanism in a transmission countershaft to mitigate idle rattle.
U.S. Pat. No. 4,677,868 discloses an idle rattle mechanism incorporated in a countershaft assembly of a manual transmission of the general type disclosed herein. The countershaft assembly includes a cluster gear having ratio gears fixed thereto, a driven or head gear journaled on the cluster gear, loosely intermeshed teeth fixed to the cluster gear and the driven gear to limit relative rotation therebetween, and a viscous liquid disposed between the teeth for damping engine idle torsionals which cause idle rattle. This mechanism is not believed to be effective for reducing in-gear rattle, body boom, or vehicle jerk.